This is an extruder wiper. I was tired of cleaning each time the extruder with tweezers before printing and having a ton of strings everywhere so I copied the idea of the Cel Robox and created a wiper, so it gets clean before each printing automatically.
You can see it in action here: https://www.youtube.com/watch?v=17oLfm5vTJA
For the wiper itself I have used heat shrink rubber preheated until it shrinked at it's maximum.
Also, I have included my start script (it includes also a pre-print line to ensure good plastic flow at the beginning of the printing).